Our overall objective is to increase our understanding of the role and functioning of lysosomes. Present work is focused on a) the isolation, characterization, and comparison of three intracellular alpha-D-mannosidases of rat liver, which occur in lysosomes, in Golgi membranes, and in the soluble cytoplasm, and on b) the in vitro fusion of phagolysosomes of Acanthamoeba. The mannosidase study may shed light on the origin and function of these enzymes, and the fusion research may eventually permit us to discover cytoplasmic and membrane determinants which regulate the process.